In general terms, an apparatus of this type also includes an air transmission branch which is interposed between the air inlet chamber and the air outlet chamber, together with a regulating valve which is arranged to distribute an air stream between the air heating branch and the air transmission branch.
Arrangements of this type are already known, which are suitable for installation in either a vehicle having an internal combustion engine, or an electrically propelled motor vehicle. Where the vehicle has an internal combustion engine, the heating means of the heating and ventilating apparatus conventionally comprise a heat exchanger which is fed with engine coolant liquid heated by the engine, with an air stream passing through the heat exchanger and being delivered into the cabin. Heating of the cabin is thus obtained by recovering at least some of the heat given off by the engine.
In modern motor vehicles having internal combustion engines, the power which is available in the engine cooling circuit is becoming more and more restricted as a result of continued improvement in engine efficiency. The heat losses are often, in consequence not sufficiently large to provide effective heating, especially in cold weather. In the case of an electrically propelled vehicle which is powered from an electrical voltage source carried by the vehicle, that is to say batteries, the motor has hardly any useful heat loss.
For this reason, such vehicles are currently heated either by means of a burner, or by means of one or more electrical heating resistances which are fed by the batteries, and through which an air stream is delivered into the cabin of the vehicle by a blower. These resistors, which can be referred to as "pure resistors" or "passive resistors", give rise to thermal safety problems due to the fact that, when the flow of air is stopped, the power of the resistors is unable to be dissipated in the form of heat. This gives rise to risks of fire by melting, followed by combustion of the plastics materials located in the vicinity of the resistors. In order to overcome this drawback, it is necessary to provide a temperature detector which controls a suitable electronic power regulating device, so that all of the heat is delivered at a low temperature, the power being interrupted at high temperature, for example at temperatures greater than 120.degree. C.
In addition, these known types of heating resistor have the disadvantage that they have a high electric power consumption; and since this power is taken from the batteries, the autonomy of the vehicle (that is to say its scope for travelling independently) is thereby restricted.
It has recently been proposed to use heating resistors of the positive temperature co-efficient (PTC) type, that is to say resistors in which there is a very great variation in the value of the resistance as a function of temperature. This is by contrast with pure or passive resistors, the resistance of which remains practically constant regardless of the temperature.
In PTC resistors, the resistance value increases very sharply beyond a temperature threshold referred to as the "critical" or "transition" temperature. They have the advantage of being auto-adjustable for temperature, so that the electrical power adjusts itself to a working temperature which is almost constant in the running mode. The use of a positive temperature co-efficient heating resistor thus enables the electric power consumed to be adapted to the desired thermal power, without the need to provide any additional control means. On the other hand, PTC resistors have the disadvantage that their resistance is low on starting when the temperature is very low, for example at -20.degree. C. As a result, under these conditions there is an abnormally high electric current through the resistor, due to the very low value of the resistance of the external electric circuit (i.e. the battery and appropriate electrical connections). This very high current can be dangerous, even if it occurs only over a very short period of time.